The present invention is generally directed to electrophotography, and, in particular, is directed to a process of preparing photoconductive pigments.
Photoconductive pigments are used, for example, in photogenerating layers of electrophotographic imaging members. Most phthalocyanine pigments are derived from solid forms of phthalocyanine which are characterized by a hard crystalline structure and, in the unprocessed state, lack pigmentary qualities. Processes of milling phthalocyanine to achieve a desired pigment form are known in the art. Even if ground to a particle size of a few microns, the phthalocyanine thus processed has little tinctorial strength. To achieve the pigmentary state, further processing is required. Generally, it has been necessary to grind the phthalocyanine into a state of fine subdivision and softness, for ease of dispersion in order that it may be incorporated easily into a pigmentary medium.
Phthalocyanine exists in many forms. Forms commonly used for the preparation of pigments are the .alpha.-form and the X-form.
The .alpha.-form of phthalocyanine and metal-free phthalocyanine having a substituent on the benzene nucleus are produced by conventional methods such as those disclosed in U.S. Pat. No. 4,619,879 to Kakuta et al. This patent describes methods of producing metal-free phthalocyanine, for example, by treating those metal-containing phthalocyanines whose metal can be removed with an acid such as sulfuric acid. Such phthalocyanines include lithium phthalocyanine, sodium phthalocyanine, calcium phthalocyanine, magnesium phthalocyanine and the like. It also describes producing metal-free phthalocyanine directly from phthalodinitrile, aminoiminoisoindolenine and alkoxyiminoisoindolenine. The metal-free phthalocyanine thus produced in accordance with these conventional methods is dissolved in sulfuric acid at a temperature preferably below 5.degree. C., or converted into the sulfate and then poured into water or iced water for reprecipitation or hydrolysis, thereby providing the .alpha.-form metal-free phthalocyanine. The resulting .alpha.-form metal-free phthalocyanine produced by these methods is preferably used in the dry form, but a water paste type may also be used.
U.S. Pat. No. 3,357,989 to Byrne et al. discloses the utility of X-form phthalocyanine as a photoconductive material in electrophotography when mixed with a binder and coated on a substrate. The X-form is the most desirable form for use in preparing infrared-sensitive photogenerators.
U.S. Pat. No. 3,594,163 to Radler describes a method of preparing X-form phthalocyanine which comprises crystal conversion from .alpha.-form phthalocyanine in organic solvents seeded with small amounts of X-form phthalocyanine. In this procedure, up to 20% by weight of X-form phthalocyanine is added to particulate .alpha.-form phthalocyanine, and the mixture is converted to 100% X-form phthalocyanine by allowing a solution containing an organic binder resin to age for several hours, and then coating the solution onto a suitable substrate. The final product of this process is a hard layer comprising X-form phthaloyanine dispersed in a resin binder.
U.S. Pat. No. 4,783,389 to Trout et al. discloses a process for preparing toner particles for liquid electrostatic imaging wherein a thermoplastic resin is mixed with a nonpolar liquid at a temperature at which the resin will plasticize and liquify and below which the nonpolar liquid boils and the resin decomposes. The mixture is cooled to formed particles of the resin the nonpolar liquid, and the resin particles are then reduced in size by passing the mixture of resin particles and nonpolar liquid through at least one liquid jet interaction chamber.
A current method of effecting this conversion developed by the assignee of the present application uses a "seed" of the X-form phthalocyanine with a larger amount of the .alpha.-form. This mixture of the X-form "seed" and the .alpha.-form phthalocyanine then undergoes both dry and wet stone milling. The mixture of .alpha.-form and X-form pigment is ground in an attritor for approximately 24 hours, at which point 20-60% conversion is obtained. Conversion is then completed by forming a slurry of the mixture with a suitable solvent, and then separating the metal-free X-form phthalocyanine. In the event that methyl ethyl ketone is employed as the solvent, the period of time is 20-24 hours. The product is then washed, filter caked, and dried.
One disadvantage of this current process is the long time period required; generally this process takes about 2-3 days for 100% conversion to be effected. Additionally, the process is not readily amenable to scaling up, because of difficulties involved with performing this process on a larger scale. For example, it is possible for contaminants to be introduced during the grinding process when a foreign medium is employed to grind the material. For a large scale operation, it is also critical to utilize durable equipment which is not labor intensive in order to keep the costs of an upscale production low. It is also important to keep the processing time as short as possible to keep labor costs low and to maximize the efficiency of the equipment.
UK Patent 2,212,922A discloses a process of converting .alpha.-form phthalocyanine from X-form by subjecting a mixture of as little as 5% X-form phthalocyanine with the remainder being .alpha.-form phthalocyanine to ultrasonic radiation which effects the desired conversion in about two hours
U.S. Pat. No. 2,816,115 to Howell teaches the production of phthalocyanine coloring material wherein crude .alpha.-form phthalocyanine coloring is milled with a comminuted water-insoluble solid grinding agent, i.e. fine sand, and sufficient water to convert the mixture into a stirrable viscous mass. The milling is done in an apparatus which causes the mass to undergo internal shear rather than impact, such as a disc mill.
U.S. Pat. No. 4,507,374 to Kakuta et al. discloses a process of manufacturing a form of phthalocyanine wherein metal-free phthalocyanine is subjected to milling by agitation or with a mechanical stress at 50.degree. to 180.degree. C. Typical devices for carrying out the crystal transformation process are ordinary agitating devices, such as a homomixer, a disperser, an agitator, a stirrer, a kneader, a Banbury mixer, a ball mill, a sand mill and an attritor.
While the prior art discloses several methods of preparing phthalocyanine, it fails to provide for obtaining phthalocyanine in a relatively short period of time with minimal contamination and superior electrical properties.